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Chapter 4
PFI 0 and Digital Input Measurements
Attenuating Voltage
Transducers can generate more than 10 VDC per channel, but DAQ devices cannot read
more than 10 VDC per input channel. Therefore, you must attenuate output signals from the
transducer to fit within the DAQ device specifications. Figure 4-12 shows how to use a voltage
divider to attenuate the output signal of the transducer.
Figure 4-12.
Attenuating Voltage with a Voltage Divider
The voltage divider splits the input voltage (
V
in
) between two resistors (
R
1
and
R
2
), causing the
voltage on each resistor to be noticeably lower than
V
in
. Use Equation 4-3 to determine the
V
m
that the DAQ device measures:
(4-3)
Use Equation 4-4 to determine the overall gain of a voltage divider circuit:
(4-4)
The accuracy of Equation 4-4 depends on the tolerances of the resistors that you use.
Caution
The SCB-68A is
not
designed for any input voltages
>30 V
rms
/42 V
pk
/60 VDC, even if a user-installed voltage divider reduces the voltage
to within the input range of the DAQ device. Input voltages >30 V
rms
/42 V
pk
/60 VDC
can damage the SCB-68A, any devices connected to it, and the host computer.
Overvoltage can also cause an electric shock hazard for the operator.
R2
V
in
V
m
+
–
+
–
R1
V
m
V
in
R
2
R
1
R
2
+
------------------
=
G
V
m
V
in
-------
R
2
R
1
R
2
+
------------------
=
=
